CARBOHYDRATES 1 13 



disaccharose sugars, viz. cane sugar^ malt sugar and milk 

 sugar, when acted upon by enzymes or when boiled with acids, 

 yield glucose. Starch also by similar reactions is broken down 

 into glucose, and this is the commercial method of preparing it. 

 These reactions will be more fully considered later. 



Fermentation of Glucose. — By far the most important re- 

 action of glucose is its fermentation. When a solution of glu- 

 cose is acted upon by the plant organism yeast (saccharomyces) , 

 it yields alcohol and carbon dioxide. 



C6H12O6 + yeast (zymase) -> 2 C2H5OH + 2CO2 



This fermentation, as has been fully described under alcohol, 

 is due to an enzyme known as zymase, which is secreted by the 

 yeast cell. Alcoholic fermentation takes place in all glucose- 

 containing solutions whether the glucose is present in such 

 solutions naturally, as in grape juice and apple juice, or whether 

 it has been formed by a preceding decomposition of malt sugar 

 or starch, as in the case of malted grain. 



Fructose or Levulose, C6H12O6 



Fructose or levulose is also known as fruit sugar, and is found 

 usually associated with glucose in fruits such as grapes, and in 

 honey. As its name, levulose, signifies, it is oppositely active 

 toward polarized light to dextrose, being levo-rotatory. It 

 does not crystallize easily, being more soluble than glucose both 

 in water and in alcohol. It is like glucose in fermenting with 

 yeast zymase, though less rapidly, and in reducing Fehling's 

 solution. In their constitution glucose and fructose differ in 

 that the former is an aldehyde-alcohol compound and the latter 

 a ketone-alcohol compound. In the decomposition of cane 

 sugar by enzymes or by boiling with acids not only glucose but 

 also fructose is formed, and in equal molecular amounts, one 

 molecule of glucose and one of fructose. In the decomposition 

 of starch and malt sugar, however, only glucose is formed. 

 Fructose is the direct product of a laboratory synthesis of sugars. 

 Formaldehyde when treated with alkali is converted into this 



